Telephone system



u y 1943 c. M. MORRIS 2,324,317

TELEPHONE SYSTEM Filed March 27, 1942 //vv/v TOR C. M. MORRIS Patented July 13, 1943 TELEPHONE SYSTEM Charles M. Morris, East Orange, N. 1., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application March 27, 1942, Serial No. 436,440

6 Claims.

This invention relates to telephone transmission circuits and, more particularly, to a circuit arrangement for minimizing interfering disturbances, such as crosstalk, in the operation of telephone communication circuits, for example, of the type in which three or more key telephone sets or equipments are associated with a plurality of telephone lines.

It is an object of the invention to reduce crosstalk between communication circuits or lines of the type in which the ringing circuit and talking circuit capacitors of each telephone set or station associated with a plurality of telephone lines are housed in a single container, and in which operating conditions may necessitate the electrical association of a capacitor of one line with a capacitor of another line.

A more particular object of this invention is to reduce such crosstalk in a circuit arrangement comprising a pair of telephone lines each connecting with a central ofi'ice or exchange, at least three or more key telephone sets being associated with such lines for selective connection with either, each set including the above-mentioned ringing circuit condenser and speech currents circuit condenser arrangement.

In what are known as combined hand telephone sets, it is the practice to enclose and to support all of the subscribers station equipment, such as the speech currents induction coil and condenser, and the ringer and its associated condenser, in the base of the telephone set. To conserve space further, it has been found desirable to enclose the two condensers in a single metallic container. The latter construction results in certain natural internal capacitances, resulting from the direct capacity between the two condensers, and the capacities between each condenser and the common container. These internal capacities must be considered in the provision of certain telephone facilities such as those furnished by key telephone sets.

A key telephone set includes facilities that enable a subscriber to use the telephone equipment at his station in answering and in initiating calls on a telephone line or lines other than the telephone line to which his set or equipment is normally connected. In order that the subscribers line from which the station equipment has been transferred while a call on a second line is being handled, be made available for use by another subscriber or subscribers having the same facilities, it has been found practicable to interpose the switching equipment, or keys between the ringer bridge and the speech receiving and transmitting equipment of each line so that when the keys are operated, the ringer circuit at each set is undisturbed in so far as its connection to a particular line is concerned, and only the speech receiving and transmitting circuits are switched to the other line or lines. It follows, therefore, that under such conditions wherein subscribers are using lines other than the ones to which their speech circuit equipment is normally connected, the ringing condenser of one line is electrically associated with a speech circuit capacitor of a second line. Because of the capacitive coupling between capacitors housed in the same container but electrically associated with different lines, the possibility of crosstalk between the two lines is present.

M. A. Logan Patent 2,230,357 of February 4, 1941 treated of this general problem in so far as it was presented by a circuit arrangement comprising two telephone lines each connecting with a central oflice or exchange and two key telephone sets, each set normally associated with one line but adapted through its key or keys to be transferred to the second line. With one set idle and the other set in use and connected with either of the two lines, the matter of crosstalk is not a concern. With each set in use and connected to the line with which its associated ringer circuit is not connected, crosstalk may exceed the permissible maximum level. By appropriate proportioning of the capacitances and by connecting the condenser container to a particular line terminal of the induction coil of each key telephone set, the patentee attained a balanced circuit that minimized or balanced out crosstalk It has been found, however, that when one or more additional key telephone sets are associated with such a pair of telephone lines, each additional set being adapted to be connected for responding to or initiating a call over either of the two lines, a crosstalk condition may be presented that is not effectively protected against by the patented arrangement. This condition is one that exists when two sets are in use, one on each of the two lines, the idle set or sets providing a capacitive coupling between the two active sets.

In accordance with this invention, crosstalk in such a circuit arrangement involving two telephone lines and three or more key telephone sets, is minimized or reduced by conductively connecting the common condenser container in each set to the ring conductor terminal of the ringer circuit of the set, that is, to the line ring conductor on the line side of the key or keys in the set.

A more complete understanding of the invention will be obtained from the detailed description that follows, taken in conjunction with the single figure of the appended drawing.

The drawing shows a key equipment telephone circuit comprising a pair of telephone lines or signaling and communication paths [0, 20, each comprising a so-calledtip conductor or wire H, 2| and a so-called ring conductor or wire [2, 22. A plurality, specifically, three, key telephone sets 30, 40, 50 are associated with the lines Iii, 23, each set being adapted to be connected to the lines I0, 20 over the connecting wire pairs 3|, 4|, 5| and 32, 42, 52. The three sets may be identical and comprise a transmitterT, a receiver R, a three-winding induction coil 10, a-ringer H, a ringing circuit condenser 12, a speech currents circuit condenser 13, two pairs -'14, of speech currents circuit contact springs, and two keys KI, K2 for selectively connecting the speech currents circuit of the set with either telephone line. The transmitter and receiver may be included in a handset or hand telephone set. The condensers 12, 13 are housed in a single metallic container 16. All of thecomponents of the set may be mounted on and/orin a single support or enclosure, that is, it may be of the so-called combined set type. Each set is shown in its idle condition, that is, with the handset on its support and spring pairs 14, '15 open. Whenthe handset is removed from its support,spring pairs 74, 15 close; the transmitter T in series with one winding 11 of the induction coil, is connected through spring pair 14 between terminalsLl L2; induction coil windings 18, 19 are connected in series with the condenser 13 and in bridge of the transmitter; and the receiver is connected through spring pair 15 in bridge of induction coil winding 18. In the idle condition of the set, the keys Kl, 'K2may beas shown, or either keyKl or K2 may have been'actuated so that its inner pairof contacts are open and its outer pair of contacts are closed, the terminals Ll, L2 in'the latterevent being connected'through either key KI and wires 3| with line), or key K2 andwires 32 with line H. In the case of sets 30, 40, the ringer H and condenser 12 are connected in series permanentlybetween terminals RI, TI in wire .pairs'3l, 4l, and in the case of set 59, the ringer and ringer condenser are connected in series permanently between terminals RI Tl in wire pair 52.

In the showing of set areindicated the natural capacitances CRO, Crc, CRT, where CRO is the capacitance between the condenser'12 and the container'IS, C'Tc is the capacitance between the condenser 13 and the container 15, and CRT is the capacitance between the two condensers.

Since the ringing circuits of sets 30, 40 are con- 'nected with line IE, these sets are normally adapted and intended to receive any call incoming on that line, and since the ringing circuit of set 'is connected with line 20, the latter set is normally adapted and intended to receive any call incoming on the latter line. On outgoing 'calls tothe central office, sets 30, 40 would normally use line It), and set 50, line 20. The keys K2 at sets 3!], 4!! enable these sets to be connected with line 20 so as to answer an incoming call or to make an outgoing call on that line, and the'key Kl at set 59 enables the latter to answer an incoming call or to make an outgoing call on line Hi. It is assumed that the set 50 would be so located with respect to the sets 30 so that the user or attendant at each set could hear the ringer at the others. If it is assumed that a call is incoming on line 20 and the set 50 does not respond, either because engaged on a call on line ID or because the attendant is not at the set, either set 30 or 40, by operation of key K2 could take the call. If, on the other hand, either set 30 or 4B is using line ID or line 20, and there is an incoming call on the line not in use, set 50 .and either set 46 or 50, as the case might be,

could answer the call by operating the appropriate key Kl or K2. In any event, there might and would be occasions when two of the three sets are connected with the lines I0, 20 and in use for telephonic purposes. In such a situation, it is desirable that crosstalk between the two sets in use, because of the coupling between them effected by the idle third station, .be prevented or be maintained below the permissible maximum.

In the above-noted Logan Patent 2,230,357, there is disclosed a telephone system comprising two telephone lines with which are associated two key telephone sets, each set adapted to be selectively connected with either line. Each set includes a ringing circuit condenser and a talking circuit condenser in a common metallic container, the metallic container being conductively connected to that terminal of the sets induction coil that is connected to the ring conductor of the line. With this arrangement and appropriate proportioning of the condensers and their capacitances with respect to one another and their metallic container, crosstalk between the sets through the ringing circuit condensers permanently connected across the lines, is minimized or balanced out.

Where three or more key telephone sets'are associated with two lines and two of the, 'say, three sets are in use, it has been found that if the Logan practice were followed, crosstalk between "the two sets-inuse may be of undesirableor objectionable level because the two sets maybe capacitively coupledthrough the ringing circuit condenser of thethird, or more, and idle, sets.

Inaccordance with the'present invention, the conductive connectionfrom the metallic container 16 for the condensers in each set is brought out to the ring conductor terminal RI of the ringer'in each set, rather than to the terminal Ll.

Let it be assumed for purposes of discussion that the conductive connection 80 were made from the container 16 to the terminal Ll ineach set instead of to the RI terminal as shown. Consider, then, the condition existing'when station 30 is in its idle condition, that is,with contact pairs 14, 15 open,'and with its key K2 operated so that the set 30 is connected to line 20; and with sets 41] and 50 in use, with key Kl of 'set 40 operated so that its speechcircuit is connected with line Land with key K2 ofset 50 operated so that the speech current circuit of the latter is connected with 20. Under such con ditions, the capacitance CRC would be a source of coupling between the conductor ll of line 10 and the conductor 22 of line 20, thepath being from conductor ll over one Wire 3| to terminal TI to one terminal of condenser 12, capacitance CRO, connection to terminalLLright inner contact of key Kl, right outer contact of key K2, over one wire 32 to conductor 22. Since the contact pairs 74, 15 would be 0Den,,the terminalLLZ would have substantially the same potential as the terminal LI, and capacitance CRT would be in parallel with capacitance CRO thereby increasing the coupling between'the conductors Hand 22, CapacitanceCTc would appear as a shunt across the induction coil and would have a negligible effect on the amount of crosstalk. The unbalanced capacitance between lines I0, 20 on which sets 40, 50 are talking would be CRC+CRT=CXT and for N such idle stations associated with lines of, say, 600 ohms impedance, the crosstalk at 1000 C. P. S. would be of the order of .48 NCx'r crosstalk units. Assume, now, that set 30 is one of the two sets in use, of which the other is set 40, set 30 being connected to line 20 through key K2 and set 40 to line I through key KI. Capacitance CRT is now a source of coupling between conductor II of line I0 and conductor 2| of line 20, capacitance CEO is again a source of coupling between conductor I I of line I 0 and conductor 22 of line 20, the unbalanced capacitance being CRTC'Rc=CXT. As before, capacitance Crc appears as a shunt across the induction coil with only a negligible effect on crosstalk. In crosstalk units, therefore, the crosstalk will be .48 NCxT.

Mimimum crosstalk coupling will result if in the idle and the talking conditions of the sets, the intercapacitances between lines I0, 20 are arranged so that approximately balanced capacitance coupling is provided. This is achieved with the arrangement shown in the drawing in which the conductive connection 80 from the condenser container is to the terminal RI of the ringer and by proportioning the condenser separations from each other and the container such that capacitance CRT is approximately equal to capacitance C'rc. Capacitance CRC appears as a shunt of negligible effect across line I0. Capacitances CRT and GT0 are effective in coupling lines I0, 20 but in opposite phase. There are four arrangements of two capacitances which will result, in a circuit arrangement as shown in the drawing, in balanced capacitance coupling: (1) conductor I I to conductor 2I and conductor I I to conductor 22; (2) conductor I2 to conductor 2| and conductor I2 to conductor 22; (3) conductor II to conductor 2i and conductor I2 to conductor 2|;

(4) conductor II to conductor 22 and conductor I2 to conductor 22.

With connection 80 made to terminal RI, arrangement (3) obtains under the talking condition of the set, and arrangement (4) under the idle condition of the set. Since capacitances CRT and GT0 couple the conductors I I, I2 to terminal L2, the potential to ground at the latter point, with respect to the metallic voltage on line I0, is proportional to CRT-CTC, and it is immaterial from a crosstalk standpoint whether the current from terminal L2 enters the line 20 on either conductor 2|, as in the talking condition of the set, or on conductor 22, as in the idle condition of the set. In either event, no crosstalk current can flow in line 20 if CRT=Crc.

As already explained, if the container I6 is connected to terminal L, arrangement (1) would exist for the talking condition of the set with an unbalanced coupling obtaining under the idle condition. If the container 16 were connected to L2, instead of to LI or RI, CTC would be short circuited, Car-i-C'zzc would couple TI to L2, and none of the four arrangements are obtained under either the idle or the talking condition of the set. A result similar to the latter follows if the container is connected to TI.

Although this invention has been described with reference to a specific embodiment, it is not limited thereto but is of a scope evidenced by the appended claims.

What is claimed is:

1. A subscribers telephone set comprising, in combination, a speech transmitting and receiving circuit and a signal circuit, a metal container, a condenser included in said speech circuit and housed in said container, a condenser included in said signal circuit and housed in said container, said condensers being so separated from each other and from the container that the capacity between said first condenser and said container equals the direct capacity between the condensers, and a connection from said container to a terminal of said signal circuit whereby the internal capacity between said second condenser and said container is effectively connected across the terminals of the signaling circuit.

2. A telephone system comprising a telephone line, and a sub-scribers telephone set connected to said line; said telephone set including a signal circuit permanently connected to said line, a speech currents circuit normally disconnected from said line but adapted to be connected thereto through circuit opening and closing means, a condenser included in said signal circuit, a condenser included in said speech circuit, a metallic container constituting a single enclosure for said condensers, said condensers being so separated from each other and said container that the capacity between the speech circuit condenser and the container and the capacity between the two condensers are substantially equal, and a connection from said container to a terminal of said signal circuit whereby the capacity between said signal circuit condenser and said container is effectively connected across the line.

3. A telephone system comprising a first telephone line, a second telephone line, and at least three telephone sets, each set including a speech transmitting and receiving circuit and a signaling circuit and each of said circuits including a separate condenser, the two condensers of a set being housed in a common metallic container, the signaling circuit of one set being permanently connected to said first line, the signaling circuit of a second set being permanently connected to said second line, and the signaling circuit of the third set being permanently connected to either of said lines, means for connecting the speech circuit of any set to either of said lines whereby the condensers in the same container are electrically included in different lines and the lines are thus electrostatically coupled, and means for reducing crosstalk resulting from such coupling, such latter means comprising a conductive connection between each of the containers and a line conductor of the line to which the associated signaling circuit is connected, said conductive connection being independent of the means for connecting the associated speech circuit to the lines.

4. A telephone system comprising a first telephone line, a second telephone line, and at least three telephone sets, each set including a speech transmitting and receiving circuit and a signaling circuit and each of said circuits including a separate condenser, the two condensers of a set being housed in a common container, the signaling circuit of one set being permanently connected to said first line, the signaling circuit of a second set being permanently connected to said second line, and the signaling circuit of said third set being permanently connected to said first line, means for connecting the speech circuit of a set,

independently of its associated signaling circuit, with either the first line or the second line whereby the condensers in the same container are electrically included in different lines and the lines are thus electrostatically coupled, and means for reducing crosstalk resulting from said coupling, such latter means comprising a conductive connection between each of said containers and a line conductor of the line to which the associated signaling circuit is permanently connected, said conductive connection being independent of the means for connecting the associated speech circuit with said lines.

5. A telephone system comprising a first telephone line, a second telephone line, and at least three telephone sets, each set including a speech transmitting and receiving circuit and a signaling circuit, and each of said circuits including a separate condenser, the two condensers of a set being housed in a single metallic container, the signaling circuit of one set being permanently connected to said first line, the signaling circuit of a second set being permanently connected to said second line, and the signaling circuit of the third set being permanently connected to said first line, means for connecting the speech circuit of a set with either of said lines, the speech circuits of said first and second sets when in use for telephonic communication and connected to the lines to which their associated signaling circuits are permanently connected, being electrostatically coupled through the signaling circuit'of the third set in the event the speech circuit of the idle third set is connected with said second line, and means for reducing crosstalk resulting from said coupling, such latter means comprising a direct conductive connection between each of the condenser containers and a line conductor of the line to which the associated signaling circuit is connected.

6. A telephone system comprising a speech transmitting and receiving circuit, a telephone line for connecting said circuit with an exchange,

a second telephone line leading to an exchange,

circuit opening and closing contacts for interconnecting said speech circuit and either of said lines, a second and a third speech transmitting and receiving circuit having individual circuit opening and closing contacts for interconnecting said second and thirdspeech circuits with either of said lines, a signal circuit individual to each speech circuit and connected on the line side of the circuit opening and closing contacts, each speech circuit including a condenser and each signal circuit including a condenser, the condensers of each speech circuit and its associated signal circuit being included in a unitary structure comprising a metallic container, and a conductive connection from the container to one line conductor on the line side of the contacts individual to the speech circuit.

CHARLES M. MORRIS. 

